A known conventional seat heater is a seat heater which is installed respectively in a seat section and a back section of a seat of a vehicle, where electric heater lines are installed on the entire surface of a base material in a zigzag design at a predetermined pitch, between the surface covering and the main pad. In the case of such a conventional seat heater, the seat heater heats up when the electric heater lines are turned ON, and thereby a user sitting on the seat can be comfortably warmed even during the cold season.
FIG. 4 is a circuit diagram of a conventional seat heater. The seat heater shown in FIG. 4 comprises a heater for heating the seat section 401a, a heater for heating the back section 401b, an overheating-prevention unit 402, and a heating unit 403. The overheating-prevention unit 402 is heated by the heating unit 403, and turns the heaters 401a and 401b OFF if the temperature reaches a predetermined temperature.
In the case of the conventional seat heater, turning the heaters 401a and 401b ON/OFF is controlled by such a temperature control unit (not illustrated) as a thermostat or controller. If control of turning the heaters 401a and 401b ON/OFF is interrupted due to failure of the temperature control unit, for example, the temperature of the seat section and the back section increases, exceeding the predetermined temperature.
Therefore in the conventional seat heater, the overheating-prevention unit 402 turns the heaters 401a and 401b OFF if the temperature of the heaters 401a and 401b exceeds the predetermined temperature, so as to ensure safety.
A return type temperature switch is used as the overheating-prevention unit 402 of the conventional seat heater. Because even if the temperature in the interior of the vehicle becomes high and the overheating-prevention unit 402 turns the heaters 401a and 401b OFF, the heaters 401a and 401b can be turned ON again when the temperature in the interior of the vehicle returns to normal temperature (e.g. Patent Document 1).
Recently hybrid vehicles and electric vehicles have started to be popular, however in these vehicles the exhaust heat of the engines is low. Therefore in these vehicles, heating the interior of the vehicle is becoming more difficult compared with vehicles that use only the engine as the power source. Therefore a seat heater which directly warms the human body is receiving attention, and further improvements in performance, particularly improvement in the warmup performance, are demanded. Hence higher output (higher power consumption) and higher density of heater wiring are necessary.
However in the case of the seat heater according to Patent Document 1, the overheating-prevention unit 402 is a return type. This means that even if the heaters 401a and 401b are turned OFF because a temperature abnormality is detected, the overheating-prevention unit 402 turns the heaters 401a and 401b ON again when temperature drops thereafter. Therefore if the output of the conventional seat heater is increased, temperature rises, and the comfort and safety of the driver (passenger) diminishes, which is a problem.
Patent Document 1: Japanese Patent Application Laid-Open No. 2003- 109721